The Anglo-Boer War or Second Boer War was fought between 1899 and 1902 by the British Empire and two Boer states, the South African Republic and the Orange Free State.
At the beginning of the conflict the British were overconfident while the Boer states were very well prepared. In the end, the British managed to defeat the Boers and the two states were incorporated to the Union of South Africa, part of the British Empire, in 1910.
The Anglo-Boer War was fought between the "Anglo", the is the English - this means the Kingodn of Great Britain and Ireland and between the "Boer", that is Dutch settles in the area.
The Portuguese and Black Africans were not directly involved in this war.
Answer: there were two immediate ones: America's support the ongoing struggle by Cubans and Filipinos against Spanish rule, and the mysterious explosion of the battleship U.S.S. Maine in Havana Harbor.